J.T. OVERPECK / Science v.311 24mar2006

Sea-level rise from melting of polar ice sheets is one of the largest
potential threats of future climate change. Polar warming by the year 2100 may
reach levels similar to those of 130,000 to 127,000 years ago that were
associated with sea levels several meters above modern levels; both the
Greenland Ice Sheet and portions of the Antarctic Ice Sheet may be vulnerable.
The record of past ice-sheet melting indicates that the rate of future melting
and related sea-level rise could be faster than widely thought.

Millions of people and their
infrastructure are concentrated near coastlines and are thus vulnerable to
sea-level rise (1); entire countries may be submerged by a rise of a few meters.
The Intergovernmental Panel on Climate Change (IPCC) Third Assess-ment (TAR) (2)
suggested a rise of 0.09 to 0.88 m by the year 2100 unless greenhouse gas (GHG)
emissions are reduced substantially. The IPCC TAR also suggested that continuing
GHG emissions could trigger polar ice-cap melting beyond 2100, with sea-level
rise in excess of 5 m within the next millennium(2). More-recent modeling (3)
indicated that the Earth will be warm enough by 2100 to melt the Greenland Ice
Sheet (GIS) over the next millennium or so, and more-recent theoretical
considerations (4) suggested that the melting could be faster and hence more
challenging for society.

Ongoing Arctic warming is already melting ice, including
sea-ice thinning and retreat and also enhanced melting of the GIS (5). These
changes, coupled with recent changes in western Antarctica (6, 7) and the
enormous potential market and nonmarket costs of large sea-level rise, led us to
reexamine the climate associated with the last major sea-level rise above modern
levels that occurred in Earth history. Corals on tectonically stable coasts from
the last interglaciation period (LIG) provided strong evidence that sea level
was 4 to 96 m above present levels during a sea-level high stand that likely
lasted from 129,000 ± 1000 years ago to at least 118,000 years ago (813). Our
goal is to untangle the causes of this past sea-level change in order to
understand how sea levels may change in the next 100 years and beyond.

Recent assessments of the LIG climate and the sea-level high
stand have pointed mostly to the likelihood that melting of the GIS contributed
2 or more m of sea-level equivalent at that time (14, 15). This hypothesis is
supported by the substantial orbitally driven excess of North-ern Hemisphere
summer insolation 130,000 years ago relative to the present day, with no
corresponding Antarctic excess (Fig. 1) (16). More-recent work (17) coupled new
climate and ice-sheet modeling with Arctic paleoclimatic data to make a strong
case that the central part of GIS was intact throughout the LIG and that the GIS
and other Arctic ice fields likely contributed 2.2 to 3.4 m of sea-level rise
during the LIG. The low-end (4 m) estimate of observed LIG sea-level rise can
thus be explained by the high-end possible contribution from the GIS, Iceland,
and other Arctic ice fields (17), plus additional small contributions from
Northern Hemisphere mountain-glacier melt. However, the low to midrange
estimates of LIG Arctic sea-level contribution imply at least some Antarctic
contribution, and an important Antarctic contribution is required to explain a
total observed LIG sea-level rise in the 4 to >6 m range.

Recent thinning along the margins of the East Antarctic Ice
Sheet (EAIS) and, in particular, over notable portions of the West Antarctic Ice
Sheet (WAIS) (1820) suggests there may have been LIG contributions to sea
level from both parts of the Antarctic Ice Sheet. There has been longstanding
concern regarding the potential for rapid WAIS collapse and the <5-m sea-level
rise that might follow warming-induced loss of buttressing ice shelves (21).
That concern was downplayed subsequently but has reemerged because
warming-induced ice-shelf reduction along the Antarctic Peninsula and in the
Amundsen Sea region was followed by accelerated flow of tributary glaciers (6,
7, 20). Although state-of-the-art knowledge of ice sheet dynamics may not be
sufficient to simulate current or future changes in the WAIS (6, 20), our
inference that the Antarctic Ice Sheet likely contributed to sea-level rise
during the LIG indicates that it could do the same if the Earth's climate warms
sufficiently in the future.

Fig. 1. Comparison
of insolation anomalies (16) over the past 150,000 years for 70'N (top),
50'N (middle), and 80°S (bottom). Insolation sufficient to begin major
melting leading to the last interglaciation occurred only after ca.
135,000 years ago (line labeled A);an
inference based on the observation that major melting over the more
wel-constrained and re-cent deglaciation did not begin until the same
level of insolation was reached at ca. 15,000 years ago (30) (line labeled
C).A much higher rate of
Northern Hemisphere summertime insolation increase existed over the
penultimate deglaciation (line labeled B, ca.
130,000 years ago) than over the most recent deglaciation (line labeled D,
ca. 12,000 years ago).

Although the low-end (<4 m) estimates of the
observed LIG sea-level high stand may not require a substantial contribution
from the Ant-arctic Ice Sheet, there are two lines of evidence that support a
WAIS contribution (in addition to the evidence that LIG sea level may have been
substantially more than 4 m above present). First, diatom and 10Be
data collected from sediments below the ice-stream region of the Ross Embayment
indicate that the central WAIS was likely smaller at some point in the last
several hundred thousand years (22), and it now appears that the LIG, not an
earlier interglaciation [i.e., marine isotope stage 11, circa (ca.) 400,000
years ago (23)], is the most likely candidate for an associated sea-level rise
of the needed magnitude. Second is the evidence from multiple ice cores (2426)
that some process caused substantial (2.5° to over 5°C) warming over East
Antarctica beginning at the same early LIG time as the observed sea-level high
stand [as suggested by the coincidence of the peak isotope-inferred LIG warming
and CH4 levels (26)]. This is surprising given the lack of a positive
summertime south polar insolation anomaly (Fig. 1) and simulated (27) LIG
cooling over Antarctica (Fig. 2). A possible explanation is the presence of a
much-reduced WAIS that would have lowered albedo and altered atmospheric
circulation over a large area of Antarctica. These changes could have driven the
observed warmer temperatures over the Antarctic region in Southern Hemisphere
summer.

Assuming that the GIS and WAIS both may have contributed to
the LIG sea-level high stand, we used a state-of-the-art coupled
atmosphere-ocean climate model to simulate the climate of 130,000 years ago and
then compared this simulation with simulations of the next 140 years made with
the same model to learn how much sea-level rise might be expected in the future
(27). Results of our LIG climate simulation are in good agreement with observed
Northern Hemisphere warming for the LIG (17) and reveal several key
aspects of the LIG climate (Fig. 2). First, the simulated LIG was warmer than
the present period in the Northern Hemisphere but not in the Southern,
consistent with strong northern but near-zero southern summer insolation
anomalies at that time (Fig. 1). This result indicates that sea-level rise at
130,000 to 128,000 years ago probably started first with the melting of the GIS
and not the Antarctic Ice Sheet. Simulated summertime LIG warming of Greenland
is less than 5°C everywhere and averages less than 3.5'C above modern
temperatures (Fig. 2), providing our estimate of the warmth needed to cause the
shrinkage of GIS that occurred during the LIG. These temperatures were
associated with a simulated net annual reduction in snowfall over Greenland
(Fig. 2). Simulated summer sea ice in the Arctic Ocean was greatly reduced at
ca. 130,000 years ago, in accord with the paleoenvironmental record from this
region [references in (17)]. Lastly, because of the latitudinally asymmetric
insolation anomalies during the LIG, simulated annual average global temperature
was not notably warmer than present, implying that sea-level rise due to ocean
expansion at that time was likely minimal.

Comparison of the summer-season warmth sufficient to have
melted much of the GIS 130,000 years ago with simulated future climate (Fig. 2)
indicates that at (or before) 2100 A.D. (and three times the amount of
preindustrial CO2), the high northern latitudes around Green-land
will be as warm as or warmer than they were 130,000 years ago and hence warm
enough to melt at least the large portions of the GIS that apparently melted
during the LIG (17). This finding assumes that GHG concentrations will rise at a
rate equivalent to 1% per year through the end of this century; slowed increases
would delay the ice-sheet response, and faster in-creases would accelerate the
response. As with our paleoclimate LIG simulation, it does not appear likely
that increased snowfall (Fig. 2) or ocean circulation changes (17) will
offset GIS melting.

Recent assessment of future climate change (2) indicates that
the amount of future warming is highly dependent on the model used, with some
models less sensitive to elevated atmospheric GHG concentrations than others.
The model we used has midrange sensitivity and appears reasonably accurate (27).
Both past and future simulations are characterized by large Arctic warmings
(i.e., to above freezing) that extend from the spring into the fall. The future
susceptibility of the GIS to melting is also likely to be exacerbated by
soot-induced snow aging (28), a factor that probably did not play a role 130,000
years ago. Lastly, Greenland could be much warmer by 2130 than it was during the
LIG (Fig. 2), assuming a 1% per year increase in CO2 or equivalent
GHGs. Thus, by any ac-count the GIS could be even more susceptible to melting in
the near future than it was 130,000 years ago.

Recent rates of sea-level rise (2.6 ± 0.04 mm/ year) (29)
are already nearing the maximum average rate (3.5 mm/year) projected to occur
over the next 1000 years by the IPCC (2). This anticipated rate is substantially
less than the 11 mm/year average rate of sea-level rise measured for the last
deglaciation between 13,800 and 7000 years ago (30). As mentioned earlier,
however, the penultimate deglaciation, culminating with the LIG sea-level high
stand 4 to >6 m above that of the present day, was driven by a substantially
larger northern high latitude summertime insolation anomaly (Fig. 1). It seems
likely, therefore, that ice-sheet melting leading to the LIG sea-level rise
should have been at least as fast as the sea-level rise (11 mm/year) associated
with the close of the last glacial period. Although a well-constrained record of
sea-level rise leading to the LIG high stand is not yet available, there is
well-dated yet controversial coral evidence that sea-level rise over this
interval may have occurred at rates higher than 20 mm/year, perhaps right up to
the LIG sea-level high stand (31). This makes sense given the much higher
insolation (and warming) anomaly at this time and also the very real possibility
that a LIG shrinkage of the WAIS (21) may be required to explain the large
amount of sea-level rise above that of the present day at that time. Other
recent paleo-sea-level studies indicate that very rapid sea-level rise is indeed
possible (32).

Fig. 2. Simulated
climate for each of four time periods, from left to right: present day (Modern),
130,000 years ago (anomalies from present day,
LIG), 2100 A.D. (the time
atmosphere reaches three times preindustrial CO2 levels, climate
anomalies from present day, D AD 2100), and 2130 A.D. (four times preindustrial
CO2 levels, climate anomalies from present day, D AD 2130). Shown for
each time period are peak summertime (July to August and January to February
means) surface air temperature and annual snow depth. Note significant warming
at north polar latitudes and the lack of any summer warming over Antarctic at
130,000 years ago.

Our analysis, as well as ongoing changes in coastal
Antarctica, are at least suggestive that the WAIS can indeed shrink rapidly as
originally envisioned by Mercer (21). Given that there was no positive summer
(melt-season) insolation anomaly at high southern latitudes in the several
millennia before 129,000 years ago, it appears that two factors may have led to
a LIG collapse of the WAIS (or perhaps portions of the EAIS). The first may have
been the sea-level rise associated with pre-129,000 to 128,000 years ago GIS
melting, and the second factor may have been shallow ocean warming around and
under the Antarctic ice shelves that buttress portions of the Antarctic Ice
Sheet. Sea-level rise seems to have had minor effects on the WAIS during the
most recent deglaciation (33), but perhaps the greater speed of sea-level rise
into the LIG compared with that from the Last Glacial Maximum (ca. 21,000 years
ago) played a role by reducing the ability of isostatic rebound after
grounding-line retreat to shallow subice-shelf cavities and promote
regrounding. As for the sub-surface warming of south polar oceans, our LIG
simulation showed modest (generally less than 0.5° but up to 1°C) warming in the
upper 200 m of the ocean (Fig. 3) that would have further
weakened ice shelves by thinning them from below; Shepherd et al. (34)
find that such a modest warming increases sub ice-shelf melt rates
substantially, by perhaps 5 m/year up to 10 m/year. In our simulation, this
small but notable warming was due to a positive springtime (October) insolation
anomaly driving reduced sea ice and enhanced subsurface warming; note that this
cool-season warming was not large enough to generate positive surface air
temperature anomalies over the Antarctic in summer (Fig. 2). Even more dramatic
ocean warming is likely in the future (Fig. 3), along with surface air
temperature increases (in all seasons) and continued sea-level rise that could
destabilize ice shelves that buttress the Antarctic Ice Sheet. Heat transport
beneath ice shelves is highly complex, so caution is required, but the LIG may
provide a conservative constraint on the future dynamics of the Antarctic Ice
Sheet and particularly the WAIS. Moreover, the same parts of the Antarctic Ice
Sheet may prove vulnerable even given increased precipitation [e.g., (35)].

Fig. 3.
Simulated shallow (100 m, top; 200 m, bottom) annual
mean ocean potential temperatures for each of four time periods, from left to
right: present day, 130,000 years ago (anomalies from present day), 2100 A.D.
(time atmosphere reaches three times preindustrial CO2 levels,
climate anomalies from present day), and 2130 A.D. (four times preindustrial CO2
levels, climate anomalies from present day). A ca. 2-month-long positive
insolation anomaly in austral spring (Fig. 1) fails to warm surface air
temperatures over Antarctica to above freezing but nonetheless acts to warm the subsurface shallow ocean, as well as ice shelves
of the same depth.

The ice-sheet origin of the LIG sea-level high stand in
response to relatively small warming, together with recent results showing rapid
response of ice to warming [e.g., (36, 37], pose important challenges for
ice-sheet modeling; whole ice sheet models do not yet incorporate important
physical processes implicated in these changes (6, 20). Even in the absence of
more-realistic models of ice-sheet behavior, it remains that ice sheets have
contributed meters above modern sea level in response to modest warming, with
peak rates of sea-level rise possibly exceeding 1 m/century. Current knowledge
cannot rule out a return to such conditions in response to continued GHG emissions.
Moreover, a threshold triggering many meters of sea-level rise could be crossed
well before the end of this century, particularly given that high levels of
anthropogenic soot may hasten future ice-sheet melting (28), the Antarctic could
warm much more than 129,000 years ago (Figs. 2 and 3), and future warming will
continue for decades and persist for centuries even after the forcing is
stabilized (38, 39).

References and Notes

1. R. J. Nicholls, Global Environ. Change 14, 69 (2004).

2. IPCC, Ed., Climate Change 2001: The Scientific
Basis.
Contribution of Working Group I to the Third Assessment Report
of the Intergovernmental Panel on Climate Change (Cambridge Univ. Press,
Cambridge, 2001), p. 881.